In order to compare the ability across species, Dr Claes chose to investigate smalleye pygmy sharks.

The diminuitive fish measures just 22cm long and is one of the smallest shark species.

At night the tiny sharks swim at depths of around 200m, moving down to 2,000m during the day.

During his analysis Dr Claes discovered a significant difference between the pygmy sharks and their larger cousins.

"The hormone prolactin - that triggers light from [lantern sharks] - actually had an opposite action in the pygmy shark, switching off its luminescence," he explained.

Dr Claes suggested that the apparently less advanced glow control in pygmy sharks provided a clue as to how the ability evolved.

"It is likely that the control of shark [luminescence] evolved from the [camouflage] of shallow water sharks," he told BBC Nature.

Sharks in shallow seas can fall prey to threats from above, but as a defence they are able to vary the colours on their skin to blend in with their environment.

A close up of the smalleye pygmy shark's skin shows its glow and pigmentation

By producing different hormones, they can trigger darker and lighter patches of skin and, Dr Claes explained, this is the basis for deep sea species' glowing control.

For both smalleye pygmy sharks and lantern sharks the light-emitting organs glow constantly, but by triggering the light and dark coloured skin patches over them, the sharks can effectively switch this light on and off.

"This result is very important, since it gives clues to the evolutionary pathway taken by a soft-tissue phenomenon which leaves no or extremely few fossil tracks," said Dr Claes.

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